Nonablative state changeable data storage systems, for example, optical data storage systems, record information in a state changeable material that is switchable between at least two detectable states by the application of projected beam energy thereto, for example, optical energy.
State changeable data storage material is incorporated in a data storage device having a structure such that the data storage material is supported by a substrate and encapsulated in encapsulants. In the case of optical data storage devices, the encapsulants include, for example, anti-ablation materials and layers, thermal insulation materials and layers, anti-reflection materials and layers, reflective layers, and chemical isolation layers. Moreover, various layers may perform more than one of these functions. For example, anti-reflection layers may also be anti-ablation layers and thermal insulating layers. The thicknesses of the layers, including the layer of state changeable data storage material, are optimized to minimize the energy necessary for state change and optimize the high contrast ratio, high carrier to noise ratio, and high stability of state changeable data storage materials.
The state changeable material is a material capable of being switched from one detectable state to another detectable state or states by the application of projected beam energy thereto. State changeable materials are such that the detectable states may differ in their morphology, surface topography, relative degree of order, relative degree of disorder, electrical properties, optical properties including the absorption coefficient, the indices of refraction and reflectivity, or combinations of one or more of these properties. The state of state changeable material is detectable by the electrical conductivity, electrical resistivity, optical transmissivity, optical absorption, optical refraction, optical reflectivity, or combinations thereof.
Formation of the data storage device includes deposition of the individual layers, for example by evaporative deposition, chemical vapor deposition, and/or plasma deposition. As used herein plasma deposition includes sputtering, glow discharge, and plasma assisted chemical vapor deposition.
Tellurium based materials have been utilized as state changeable materials for data storage where the state change is a structural change evidenced by a change in reflectivity. This effect is described, for example, in J. Feinleib, J. deNeufville, S. C. Moss, and S. R. Ovshinsky, "Rapid Reversible Light-Induced Crystallization of Amorphous Semiconductors," Appl. Phys. Lett., Vol. 18 (6), pages 254-257 (Mar.15, 1971), and in U.S. Pat. No. 3,530,441 to S.R. Ovshinsky for Method and Apparatus For Storing And Retrieving Of Information. A recent description of tellurium-germanium-tin systems, without oxygen, is in M. Chen, K. A. Rubin, V. Marrello, U. G. Gerber, and V. B. Jipson, "Reversibility And Stability of Tellurium Alloys for Optical Data Storage," Appl. Phys. Lett., Vol. 46(8), pages 734-736 (Apr. 15, 1985). A recent description of tellurium-germanium-tin systems with oxygen is in M. Takenaga, N. Yamada, S. Ohara, K. Nishiciuchi, M. Nagashima, T. Kashibara, S. Nakamura, and T. Yamashita, "New Optical Erasable Medium Using Tellurium Suboxide Thin Film,"Proceedings, SPIE Conference on Optical Data Storage, Arlington, Va., 1983, pages 173-177.
Tellurium based state changeable materials, in general, are single or multi-phased systems (1) where the ordering phenomena include a nucleation and growth process (including both or either homogeneous and heterogeneous nucleations) to convert a system of disordered materials to a system of ordered and disordered materials, and (2) where the vitrification phenomenon includes melting and rapid quenching of the phase changeable material to transform a system of disordered and ordered materials to a system of largely disordered materials. The above phase changes and separations occur over relatively small distances, with intimate interlocking of the phases and gross structural discrimination, and can be highly sensitive to local variations in stoichiometry.
A serious limitation to the rate of data transfer is the slow cycle time, which is, in turn, limited by the crystallizing or erasing time. Phase change data storage systems suffer from a deficiency not encountered in magnetic data storage systems. In magnetic data storage systems, a new recording is made over an existing recording, simultaneously erasing the existing recording. This is not possible with phase change media optical data storage systems. To the contrary, phase change optical data storage systems require separate erase (crystallize) and write (vitrify) steps in the "write" cycle in order to enter data where data already exists.
An important operational aspect of this problem is that the long duration of the erasing or crystallization process unduly lengthens the time required for the erase-rewrite cycle. In prior art systems, the phase change materials have an erase (crystallization) time of 0.5 micro seconds or larger. This has necessitated such expedients as an elliptic laser beam spot to lengthen the irradiation time. However, for reading and for writing (vitrifying) another spot, e.g., a round spot, is necessary. This required two optical systems. Thus, in prior art phase change systems a two laser erase-write cycle is utilized. This requires a two laser head. This is a complex system, and difficult to keep aligned. The first laser erases (crystallized) a data segment or sector. Thereafter, the data segment or sector is written by the second laser, e.g., by programmed vitrification.
In the case of magneto-optic systems, two complete disc revolutions are required per cycle, one for erasing and one for writing. This particularly limits the ability to use these prior art erasable discs in real time recording of long data streams.